dmksnnk · dmksnnk · Sep 24, 2025 · Sep 24, 2025 · Sep 24, 2025 · Sep 24, 2025
diff --git a/parallel.go b/parallel.go
@@ -80,3 +80,21 @@ func ParFilter[I any](pipe Stream[I], num int, callback func(context.Context, I)
 		ops...,
 	)
 }
+
+// ParForEach is like ForEach, but calls the callback concurrently with num goroutines.
+// It's better to use it with a buffered stream.
+func ParForEach[I any](pipe Stream[I], num int, callback func(context.Context, I) error) error {
+	for i := 0; i < num; i++ {
+		pipe.eg.Go(func() error {
+			for elem := range pipe.in {
+				if err := callback(pipe.ctx, elem); err != nil {
+					return err
+				}
+			}
+
+			return nil
+		})
+	}
+
+	return pipe.eg.Wait()
+}
diff --git a/parallel_test.go b/parallel_test.go
@@ -6,6 +6,7 @@ import (
 	"math/rand"
 	"sort"
 	"strconv"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -39,7 +40,11 @@ func TestParallel(t *testing.T) {
 }
 
 func TestParallelPipeline(t *testing.T) {
-	testFn := func(producer rheos.Stream[int], mapFn func(context.Context, int) (int, error), filterMapFn func(context.Context, int) (int, bool, error)) ([]int, error) {
+	testFn := func(
+		producer rheos.Stream[int],
+		mapFn func(context.Context, int) (int, error),
+		filterMapFn func(context.Context, int) (int, bool, error),
+	) ([]int, error) {
 		size := rand.Intn(10) + 1
 		p2 := rheos.ParMap(producer, size, mapFn)
 		p3 := rheos.ParFilterMap(p2, size, filterMapFn)
@@ -141,3 +146,96 @@ func TestParallelPipeline(t *testing.T) {
 		}
 	})
 }
+
+func TestUnitParForEach(t *testing.T) {
+	t.Run("collect items", func(t *testing.T) {
+		num := int(rand.Int31n(100) + 10)
+		producer := newProducer(context.Background(), num)
+		want := intRange(num)
+
+		concurrency := rand.Intn(10) + 1
+
+		var mux sync.Mutex // guard writing to result slice
+		var result []int
+		err := rheos.ParForEach(
+			producer,
+			concurrency,
+			func(_ context.Context, v int) error {
+				mux.Lock()
+				defer mux.Unlock()
+
+				result = append(result, v)
+
+				return nil
+			},
+		)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		sort.Slice(result, func(i, j int) bool {
+			return result[i] < result[j]
+		})
+		assertSlicesEqual(t, want, result)
+	})
+
+	t.Run("returns error", func(t *testing.T) {
+		num := int(rand.Int31n(100) + 10)
+		producer := newProducer(context.Background(), num)
+
+		concurrency := rand.Intn(10) + 1
+
+		var mux sync.Mutex // guard writing to result slice
+		var result []int
+		err := rheos.ParForEach(
+			producer,
+			concurrency,
+			func(_ context.Context, v int) error {
+				mux.Lock()
+				defer mux.Unlock()
+
+				result = append(result, v)
+				if len(result) >= num/2 {
+					return errTest
+				}
+
+				return nil
+			},
+		)
+
+		if !errors.Is(err, errTest) {
+			t.Errorf("unexpected error: %v, want: %v", err, errTest)
+		}
+	})
+
+	t.Run("context is cancelled", func(t *testing.T) {
+		num := int(rand.Int31n(100) + 10)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		producer := newProducer(ctx, num)
+
+		concurrency := rand.Intn(10) + 1
+
+		var mux sync.Mutex // guard writing to result slice
+		var result []int
+		err := rheos.ParForEach(
+			producer,
+			concurrency,
+			func(_ context.Context, v int) error {
+				mux.Lock()
+				defer mux.Unlock()
+
+				result = append(result, v)
+				if len(result) >= num/2 {
+					cancel()
+				}
+
+				return nil
+			},
+		)
+
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("unexpected error: %v, want: %v", err, context.Canceled)
+		}
+	})
+}
diff --git a/rheos_test.go b/rheos_test.go
@@ -114,6 +114,7 @@ func TestUnitForEach(t *testing.T) {
 	t.Run("collect items", func(t *testing.T) {
 		num := int(rand.Int31n(100) + 10)
 		p := newProducer(context.Background(), num)
+		want := intRange(num)
 
 		var result []int
 		err := rheos.ForEach(
@@ -123,11 +124,10 @@ func TestUnitForEach(t *testing.T) {
 				return nil
 			},
 		)
-
-		want := intRange(num)
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
+
 		assertSlicesEqual(t, want, result)
 	})
 	t.Run("returns error", func(t *testing.T) {